The present invention relates to a method for curing self-reducing agglomerates containing iron oxide and cement as a binder to achieve the mechanical strength properties required for use thereof in subsequent refining operations for the production of iron.
The development of iron bearing agglomerates began in the fifties. These agglomerates were mainly in the form of pellets, with the purpose of enabling the use of mineral fines, which the conventional methods, such as those used in blast furnaces, cupola furnaces, electric furnaces and others, could not accept as raw materials due to the fine size thereof, particularly in the case of iron ore. Some time thereafter the development of self-reducing agglomerates began. This development was characterized by the use of cold cure binders, particularly cement, lime and silica, which exhibit as the cure mechanism, mainly reactions with water (hydration) and in lesser amount with the carbon dioxide present in the air (carbonation). Those reactions, although imparting to the agglomerate the desired mechanical properties, are slow reactions, requiring between 10 to 30 days for completion, and sometimes even more depending on the weather conditions (the cure velocity diminishes with the decrease of the ambient temperature).
Some alternatives were developed to accelerate the hydration reactions referred to above, by means of treatment of the agglomerates in pressure vessels (autoclaves) by applying pressures on the order of up to 20 atmospheres and water vapor at 250xc2x0 C., as recited in U.S. Pat. No. 4,528,029, which is incorporated herein. The major disadvantage of this alternative practice is the high cost of the equipment required and the complex operating conditions, rendering the commercial application thereof difficult.
In accordance with the invention, there is provided a method for producing self-reducing agglomerates for use in producing iron. The method includes producing a particle mixture comprising iron oxide containing particles of a carbon containing reductant and a cement binder. Water is added to the particle mixture to produce a moisture content therein of about 7 to 12%. Thereafter, the particle mixture is formed into agglomerates. The agglomerates are sequentially and continuously pre-dried by contacting them with hot gas at a temperature of about 80 to 180xc2x0 C. to reduce the moisture content of the agglomerates. The dry agglomerates are then contacted with water at a temperature of about 70 to 110xc2x0 C. to promote a curing reaction between the water vapor and the cement. They are then dried with hot gas at a temperature of about 80 to 180xc2x0 C. to achieve a selected moisture content.
The particle mixture may include a fluxing agent.
The agglomerates may be used to constitute a descending column thereof during sequential and continuous pre-drying, curing, and drying.
Preferably, the moisture content of the particle mixture may be about 8 to 10%.
The sequential and continuous pre-drying, curing, and drying may be performed at atmospheric pressure.
The curing step may be conducted for about 4 to 12 hours.
The iron oxide containing particles may include at least one of iron ore and industrial residue.
The descending column of agglomerates for sequential and continuous predrying, curing, and drying, may be within a single reactor vessel.
Further, in accordance with the invention, there was provided apparatus for producing self-reducing agglomerates having a cement binder for use in producing iron. The apparatus includes an elongated, vertical curing and drying chamber. An inlet is provided at the top of the chamber for introducing a particle mixture of said agglomerates. The agglomerates comprise iron-oxide containing particles and particles of a carbon containing reductant and a cement binder, with a moisture content therein of about 7 to 12%. A predrying zone is provided in the chamber at an upper portion thereof for predrying the particle mixture. A curing zone is provided in the chamber at a mid-portion thereof for curing the cement binder. A drying zone is provided in the chamber at a bottom portion thereof for drying the particle mixture.
At the predrying zone of the chamber means are provided for contacting the particle mixture with a hot gas at a temperature of about 80 to 180xc2x0 C. to dry the particle mixture. At the curing zone means are provided for contacting the particle mixture with water-containing hot gas at a temperature of about 70 to 110xc2x0 C. to cure the cement binder. At the drying zone means are provided for contacting the particle mixture with hot gas at a temperature of about 80 to 180xc2x0 C. to dry the particle mixture.
Within the chamber means are provided for controlling intermixing of the gas from each zone of the chamber. In this regard, the gas may be metered between zones to regulate or permit intermixing. This may include means for channeling the gas introduced to the predrying zone to leave the chamber at the top thereof, channel the gas introduced to the curing zone to leave the chamber from a side outlet at the curing zone, and channel the gas introduced to the drying zone to leave the chamber from a side outlet at the drying zone.
Means may be provided for diverting hot has into contact with an exterior portion of the chamber at the predrying zone thereof. The hot gas may be directed in this regard by providing a shell surrounding the exterior portion of the chamber in spaced-apart relation thereto and a passage through which the hot gas is directed to a space formed between the exterior chamber portion and the shell.